The present invention relates to the testing of elastic properties of soft biological tissues. More particularly, the present invention provides an acoustic device and method for noninvasive measurement of the velocity of surface acoustic waves in tissues thus providing the means for estimating the shear elastic properties of tissues for the purposes of medical diagnosis.
The viscoelastic properties of biological tissues are directly related to their structural organization. It was found that bulk elastic properties of tissues are mainly determined by their molecular composition, while shear elasticity is characterized of higher levels of structure. Physiological processes in an organism may result in structural changes in tissues that can be detected by measuring their shear elastic properties.
The methods of testing the viscoelastic properties of tissues are essentially based on invasive measurements on isolated samples. Maxwell, for example, employed torsional vibrations of a sample cut in the form of cylindrical rod fixed at one of the ends (Maxwell B., ASTM Bull, #215, 76, 1956). Shear modulus was determined by measuring the magnitude of force necessary to provide a given deformation at the other end of the rod. The range of working frequencies was 0.001 to 200 kHz.
For measuring the bulk elastic properties, i.e. the ultrasonic velocity and bulk compressibility of media a great variety of methods using bulk acoustic waves in a high-frequency range were developed. For example, according to one of those methods (Nole A. W., Mowry S. C. J. Acoust. Soc. Amer., 20, 432, 1948) the speed of sound was evaluated from the measurement of time-of-flight of an acoustic pulse in the sample being tested which is immersed in a liquid between the ultrasonic transducer and reflector.
Among the known methods of noninvasive testing of bulk elastic properties of tissues is the method and device for noninvasive monitoring the instantaneous fluctuations in viscoelasticity-related properties of a living tissue (Benjamin Gavis, European Patent No. EP 0 135 325 A2, U.S. Pat. No. 4,580,574, T. 1065). The device comprises a pair of substantially parallel spaced-apart piezotransducers, one of them being adjustable with respect to the other to enable insertion and clamping of a segment of a living tissue therebetween. One transducer is connected to a high frequency generator and the other is attached through an amplifier and demodulator to a signal analyzer. The frequency of the ultrasonic resonant oscillations induced in a tissue and their magnitude are characteristic of the viscoelastic properties of the tissue.
The aforementioned method of testing the tissues makes it possible to detect certain physiological processes and particularly the changes in the microcirculation of blood. But since the subject of testing are the bulk elastic properties which may vary in tissues by no more than several percent the accuracy of such a method would not be very high, as compared to the accuracy of the methods using the shear elasticity which may change in some cases by hundreds percent, depending on the physiological state of a tissue. The method does not imply any means to control the force by which the transducers are pressed to a tissue sample placed in the gap therebetween. But such a pressure affects the properties of the samples and still increases the error of measurement. Besides, because it is often difficult to provide access to the tested tissue from the opposite sides, this technique is not applicable to the most of the body.
It is impossible to evaluate by means of this method one of the important characteristics related to the structural organization of living tissues--their anisotropy, i.e. the difference in mechanical properties in various directions.
The possibilities of testing the shear elastic properties of biological tissues were investigated in the paper (V. A. Passechnik, A. P. Sarcazyan: On the possibility of examination of the muscle contracting models by measuring the viscoelastic properties of the contracting muscle--Studia Biophysica, Berlin, Band 13, 1969, Heft 2, s, 143-150). In this work the changes in elastic properties of an isolated muscle during contraction were studied. The low frequency acoustic oscillations (450 to 1200 Hz) were excited in a sample by means of a flexural piezotransducer and received at a distance by the like piezotransducer. The tension of a muscle was measured in various phases of contraction. The modulus of shear elasticity was evaluated by measuring the amplitude and the phase of received signal.
According to the other method of testing the shear elasticity of tissues (R. O. Rotts, D. A. Christman, E. M. Buras: The dynamic mechanical properties of human skin in vivo, J. Biomechanics, Vol. 16 , #6, pp. 365-372, 1983) the shear oscillations in tissue were produced by a "recorder" (a phonograph recording cutterhead) touching the tissue surface with its contact tip (stylus). A phonograph cartridge with a stylus was used as a receiver. Measurements were conducted in the frequency range of 200-1000 Hz.
The recorder was excited by the white noise sound generator; the characteristic frequencies were estimated by means of a spectrum analyzer. The measured parameters were the velocity of propagation of shear waves and their attenuation. The authors of the cited research came to the conclusion that in the low frequency range investigated by said method, the mechanical waves excited in tissues are of shear character and are localized only the superficial layer of the tissue, i.e. in the skin, and because of that only in this range it is possible to provide the selective measurement of skin elasticity, while at higher frequencies the measurement is more difficult because of the small depth of penetration of surface waves. This conclusion is disputable. Since the velocity of shear waves in soft tissues may be 5 to 50 m/s, the wavelength in the frequency range of about 1 kHz should be about 5 to 50 mm. Since the penetration depth of surface waves cannot be much less than a wavelength, the subcutaneous structure elements and in some cases the bone tissue may affect the propagation of the waves of that range, therefore the selectivity of the method as related to measurement in skin seems rather doubtful.
The authors of the said method (R. O. Rotts et al.) do not take into account a peculiarity of surface waves such as the dependence of the velocity and attenuation of a surface wave on the direction of propagation relative to the displacement vector of a tangential oscillation excited by the transmitter on the surface of tissue. Neither was mentioned the anisotropy which is the important feature of shear elasticity in biological tissues (in particular in skin).
The structure of tissues is subject to certain changes in a wide range of clinical situations. The structural changes are closely related to the change in viscoelastic properties of tissues. The bulk elastic properties are not particularly sensitive to the structure and its anisotropy as compared to the shear properties, so the testing of shear properties will prove to be of greater value for clinical medicine and diagnosis. What is therefore needed is a method for noninvasive and highly sensitive testing of shear properties of a tissue along the chosen direction.